"my friend said it can't hurt the car and its "good to clean out the car" with high octane - once a month he said".

This is the point where you stop believing everything you're told. This is also the school of thought that has been making fuel companies rich for decades.

Octane has NO cleansing properties, and 93 has no more detergents in it than 87. Different BRANDS of gas differ in their use of detergents, which is why you'll want to stick with Top-Tier fuel suppliers if cleanliness is a concern.